This invention relates to a sonar towed array and, more particularly, to the aft drogue module for a high-strength, thin-line towed sonar array.
Generally, sonar arrays are towed up to a mile or so behind the towing vessel. The towed array is deployed by unwinding it from the spool of a winch, on which it is stowed when not in use. The array may be on the order of 1,500 feet in length and 1 inch in diameter, and is generally made in sections or modules of about 90 feet in length, which are joined together by means of an electromechanical coupling. An internal strength member runs the length of each module and functions to carry the tensile load on each module, mainly caused by drag of the array as it is towed.
The strength member in each module carries the principal accumulated stress of the array. Each strength member must be connected to a termination member to transfer the tensile load of the strength member to the electro-mechanical coupling at each end of a module. The tensile load of each module is thus successively transferred to the next forward module, until the entire load of the array is passed through the forwardmost strength member termination and electromechanical coupling to a steel tow cable and thence to a towing vessel.
Some modules are acoustic modules and contain sensitive acoustic sensors. In order to provide space for the sensors and to prevent unwanted acoustic signals from being passed to these sensors, a flat or ribbon-type strength member is employed in the acoustic modules. This ribbon-type strength member is arranged inside the walls of the tubular jacket of the module so that it does not have mechanical contact with the sensors, to minimize the transfer of unwanted acoustic signals.
Vibration isolation modules function to mechanically isolate the acoustic-type modules from vibrations of the towing vessel. This results in reducing the transfer of unwanted acoustic signals to the acoustic sensors contained in the acoustic modules. The vibration isolation modules contain no acoustic sensors, and, therefore, can use a more economical braided-rope type or round-strength member, rather than the lower-profile flat, ribbon-type strength member which provides space for the sensors.
Kevlar, a synthetic material produced by E. I. DuPont de Nemours & Co., is frequently used as the strength member in military and commercial towed sonar and sounding arrays. See U.S. Pat. No. 4,160,229 to McGough and U.S. Pat. No. 4,090,168 to Miller, et al. Kevlar is an aramid, which is the generic name for a distinctive class of aromatic polyamide fibers. Kevlar has an extremely high tensile strength and greater resistance to elongation than steel. Although Kevlar is very strong in tension, it is subject to damage by being compressed, kinked, cut or bent around a tight radius.
Uneven lengths of the Kevlar strength member ribbons, common with this type of termination, cause one of the ribbons to take a disproportionate share of the load, resulting in early failure. Also, during winching operations where the pull is not straight, the Kevlar ribbons may be bent and pulled taut over a sharp edge or a tight radius, causing damage to the Kevlar strength member.
Each strength member transfers its tensile load to a termination member, and the termination member, in turn, transfers its tensile load to the coupling so that the coupling serves as a tensile coupler. In addition, the coupling serves to house and retain the electrical plug in the receptacle by which the required electrical connections are made.
The module which terminates the after end of the towed sonar array has different requirements. It contains no electrical components and thus does not need electrical connection. The after module must be stiff enough to be able to be thrust out through an exit guide tube and, once in the stream behind the moving vessel, it must provide sufficient drag to assist in deploying the balance of the array. When the array is fully deployed, the aft drogue module must be near neutral buoyancy to maintain the proper tow profile. Also, while being towed it must dampen whipping and strumming of the aft end of the towed array. In order to achieve these needs, a novel aft drogue module is required.